FOG HEAVEN / The sun will come out tomorrow. Or maybe not. It's summer in the city, and that means gray skies.

Published 4:00 am, Friday, August 19, 2005

Photo: Frederic Larson

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FOG002_150_fl.jpg The south tower of the Golden Gate Bridge took on a reflection of it's own as the heavy fog reflected its own image. The dense fog causethe Bay Area morning commute to come to a crawl. 11/18/04 San Francisco CA Frederic Larson
The San Francisco Chronicle less

FOG002_150_fl.jpg The south tower of the Golden Gate Bridge took on a reflection of it's own as the heavy fog reflected its own image. The dense fog causethe Bay Area morning commute to come to a crawl. ... more

Photo: Frederic Larson

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What causes Fog

What causes Fog

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Summer Fog

Summer Fog

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FOG01_fl.jpg A rising sun peeked over a blanket of fog that overwhelmed the bay as spring mild temperatures continue throughout the week. San Francisco CA Frederic Larson
The San Francisco Chronicle

FOG01_fl.jpg A rising sun peeked over a blanket of fog that overwhelmed the bay as spring mild temperatures continue throughout the week. San Francisco CA Frederic Larson
The San Francisco Chronicle

Photo: Frederic Larson

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A ferry boat makes it's way North across San Francosco Bay, towards Marin County under a thick layer of fog as the sun sets, filling the sky with warm colors. 7/27/05 San Francisco, Ca Michael Macor / San Francisco Chronicle less

A ferry boat makes it's way North across San Francosco Bay, towards Marin County under a thick layer of fog as the sun sets, filling the sky with warm colors. 7/27/05 San Francisco, Ca Michael Macor / San ... more

Photo: Michael Macor

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A fog bank rolls above Alcatraz Island, the sun tries to break through, as it creeps across San Francisco Bay. 7/27/05 San Francisco, Ca Michael Macor / San Francisco Chronicle

A fog bank rolls above Alcatraz Island, the sun tries to break through, as it creeps across San Francisco Bay. 7/27/05 San Francisco, Ca Michael Macor / San Francisco Chronicle

Photo: Michael Macor

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COITSUN437a_fl.jpg Wisps of fog flirts with the Oakland Bay Bridge as summer like weather brings dense morning fog into the bay as seen from the Marin headlands. 3/6/05 San Francisco CA Frederic Larson
The San Francisco Chronicle less

COITSUN437a_fl.jpg Wisps of fog flirts with the Oakland Bay Bridge as summer like weather brings dense morning fog into the bay as seen from the Marin headlands. 3/6/05 San Francisco CA Frederic Larson
The San ... more

Photo: Frederic Larson

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FOG HEAVEN / The sun will come out tomorrow. Or maybe not. It's summer in the city, and that means gray skies.

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The rest of the country may be sweltering in the grip of summer, but the Northern California coast is deep in the fog days of August, lost in a blanket of gray gloom.

San Francisco is ground zero for the summer fog, which some people think is worse this year than ever. On Wednesday, San Francisco was colder than Anchorage, Alaska. San Francisco had a low of 52 at sunrise, three degrees colder than the Alaskan city.

As every fog-belt resident knows, the fog comes in a variety of shapes and sizes. It almost never comes in on little cat feet, as the poet Carl Sandburg said.

There is much more variety. In 1968, author Harold Gilliam identified nine different summer fog formations: wreaths and domes over Alcatraz; arches over the Golden Gate Bridge; eddies and fog falls that look like cascades over Twin Peaks in San Francisco and the Sausalito hills; surges and combers over the Peninsula and past the top of the hill in Daly City; rivers of fog at places like Candlestick Park; and the so-called fog decks, where fingers of fog skip over the bay and into Berkeley.

What residents know for sure is that the San Francisco Bay Area has three seasons: winter, summer and fog. And summer is just around the corner. On the coast, summer comes in the fall.

On most Independence Days, crowds on the San Francisco waterfront gaze at nearly invisible fireworks displays, and if the month of August had a color, it would be gray. Dawn has been visible only two of the first 18 days of August in San Francisco.

The sun hasn't come out at all for days in San Francisco's western neighborhoods; no one has seen a sunset in the Sunset District for nearly a week.

On Thursday, the marine layer -- the weather expert's term for fog -- was 2,000 feet deep over the Bay Area. Even normally sunny Contra Costa was as gray as a pessimist's heart at midmorning Thursday.

"What's the fog doing here?" physical therapist Deedee Savant wondered when she noticed downtown Walnut Creek wrapped in gloom. She usually comes to work in bright sunshine from April to November.

A finger of fog even penetrated into the Sacramento Valley on Thursday morning. More is in store: the National Weather Service is calling for fog and low clouds through next Thursday. The moon will be full tonight in the rest of the world, but in San Francisco, and on the coast from Eureka to Big Sur, the light of the moon will be only a rumor.

Point Reyes, 30 miles northwest of the Golden Gate, is the second foggiest place in America. The foggiest is the aptly named Cape Disappointment at the mouth of the Columbia River in Washington state.

Fog in the summer shouldn't be a surprise to anyone who has lived in California for more than a year, but the depth and extent of the fog always is a shock at this time of the year, especially when reports of baseball games starting in 100-degree temperatures and news of August heat filter back from the East, the Midwest and even the rest of California.

"I'm up to my nose in fog," said Helen Johnson, who has lived in San Francisco -- "in the fog," she says -- for 16 years. Johnson runs the heater in her house in the West Portal district four months of the year, she says -- in January, February, July and August.

"This has been one of the foggiest Julys and Augusts I've ever seen," she said. Johnson, a transplanted New Englander, used to fly to Boston every August to get away from the fog, only to run into heat and humidity -- "fog is just humidity in another form," she said.

"We haven't seen the sun for quite a while," said Martin Wefald, who has lived at 48th Avenue and Cabrillo Street on the western edge of San Francisco for 46 years. "I don't miss it," he said. Like many San Franciscans, he accepts the fog and is vague about what causes a foggy summer. "Global warming, maybe, or global cooling," he says. "Maybe it's the Republicans." Republicans, of course, are as rare in San Francisco as an August heat wave.

The fog is such a part of the area that it has become part of the literature of San Francisco. Poet George Sterling referred to the "cool gray city," and Dashiell Hammett made the fog -- "thin, clammy and penetrant" -- almost a literary character in his masterpiece, "The Maltese Falcon." John Steinbeck thought the San Francisco fog rolling in looked like a herd of woolly sheep, and the fog in the Salinas Valley reminded him of thin flannel. "The coldest winter I ever spent was a summer in San Francisco," a saying that is almost a San Francisco cliche, turns out to be an invention of unknown origin, the coolest thing Mark Twain never said.

As every schoolchild knows, the summer fog is caused when the sun heats the Sacramento and San Joaquin valleys, which draws in the cool marine air from the Pacific Ocean. Often a low atmospheric pressure area is associated with the interior valleys and an area of high atmospheric pressure out in the ocean.

The high-pressure area is called the Pacific High. In the meantime, the ocean currents produce a rising of cooler water. The result is condensed ocean water that rises in a mist: fog. Now the changes in atmospheric pressure move in -- air from high-pressure areas on the coast moves toward low-pressure areas, producing wind and moving the fog inland.

July was warmer -- and less foggy -- on the coast this summer. August has been more like normal -- average for this time of the season, said Jan Null, a meteorologist who is an expert on the Bay Area's climate.

"You have to think of the air as a fluid," he says, "and that means it takes the path of least resistance."

The result is that some places are foggier than others, because the fog flows like water through gaps in the hills. The Golden Gate, at sea level, produces rivers of fog, but fog comes through the Russian River Valley -- which is why Santa Rosa was foggy Thursday morning -- over the low spot in the hills near Muir Woods, over Twin Peaks in San Francisco, over the hills near Daly City and down the hills near San Francisco's Bayview district.

The most famous spot for fog in sporting history is the ballpark at Candlestick Point, where the fog and wind nearly drove the San Francisco Giants out of town. Null says that the river of fog blowing on summer evenings would hit Bayview Hill, just above Candlestick Point, and swirl, "like a rock in the middle of a river."

The fog swirls hit the stadium nearly every afternoon and evening. "If you would pick the worst place for a baseball stadium, that would be it," he said.

The Bay Area's hills, valleys and bays all produce microclimates, so one place can be foggy in August and another warm and sunny. On Thursday, for example, there was no sun at all in most of the city, but the high temperature was a pleasant 72 degrees; it can be sunny and cold in the same city on the same day.

Some places on the coast deny they have fog. In Pacifica, where a Fog Festival is held every September, the Chamber of Commerce boasts that the city has 274 days of sunshine a year. "Fog here?" says chamber president Ron Eagleson, "That's a rumor.

"Right now," he said at midweek, "I'm looking out my window at people out there in bikinis." People come to Pacifica, which stretches along the coast for miles, to escape the fog. The fog festival? "That's just a joke," he said.

Is this the foggiest August in years? Null thinks not. "I have not seen any significant changes," he says. "It's all within the realm of normal variation."

Capt. William Grieg, president of the San Francisco Bar Pilots, who pilot big ships in any weather, says this August is just "a typical summer. There's no more fog than usual."

There are those who hate the fog -- and most of them moved from the fog belt to the warmer suburbs years ago. But then there are those who love it.

"We are fog people," Jerry Pedersen, a San Francisco carpenter, said the other foggy afternoon while nipping into Flanahan's Pub on Noriega Street in the Sunset District to get out of the chill. "It's another beautiful day out here," he said. "No sun at all."

What causes fog

How fog is formed and why it moves from the coast toward inland valleys:

1 Winds around the Pacific High (a semi-permanent area of high pressure), circling clockwise due to the Earth's rotation, push ocean surface waters south and away from the coast.

2 The resulting current causes an upwelling of cold water from ocean depths.

3 The cold surface water cools the air above it, causing its moisture to condense into fog.

4 Warm air in the sun-heated Central Valley rises and is gradually replaced by cool ocean air, which draws the fog across the bay and toward the valley.

Fog forecasting on the Web

One of the best ways to see what the fog has in store is on the Web at SFGate.com's

three dimensional animated fog forecast (go to www.sfgate.com/weather). It shows the extent of the fog as predicted over the next 24 hours for both Northern California and the Bay Area. The forecast display moves hour by hour so users can see how the fog is expected to move in and out from the coast in 3-D. At right are examples from today's fog forecast.

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